1. Field of the Invention
The present invention relates to a semiconductor device provided with trenches having high and low aspect ratios respectively and a method of fabricating the same.
2. Description of the Related Art
Semiconductor devices employ a shallow trench isolation (STI) structure in order that elements may be isolated from each other. A trench is formed in a semiconductor substrate and filled with an insulating material such as silicon oxide (SiO2), whereby the element isolating STI structure is formed.
In forming a trench, a semiconductor substrate with an etching pattern is disposed in a vacuum process chamber of a processing apparatus. A reactive gas is then introduced into the chamber and a discharge plasma is generated so that reactive ions and radicals progress etching, whereby a trench is formed. A film of insulating material is formed on the trench, whereupon the STI structure is obtained. An amount and/or energy of each of ion and radical is adjusted so that an etched film is formed into a desired geometry or so that a deposited film has a high trench fill capability and high fabricating and film-forming speeds are ensured.
However, an aspect ratio of the STI region is increased as the size of a device structure is reduced. With this, the trench fill capability of an insulating film material tends to be reduced. As a result, the trench cannot completely be filled with the insulating film material, whereupon voids are formed in the trench. The voids further result in a new failure such as short circuit between the elements with an increasing frequency of occurrence. JP-A-2000-43413 discloses an improvement in the trench fill capability of the insulating film in the trench.
In order that the trench fill capability of the insulating film may be improved, it is suggested that a taper angle of STI or an inclination of sidewalls relative to the trench bottom be reduced, that is, the inclination of trench sidewalls be gentler. However, this method has the following inconvenience. That is, a distance between devices needs to be set to be shorter as miniaturization of the device structure progresses. In this case, when the taper angle is reduced in consideration of the aforementioned respect, a distance between portions where sidewalls intersect the bottom becomes shorter. Accordingly, limitations of a lower limit value of the taper angle become strict according to an opening width of the trench such that the limitations are difficult to cope with.
When the taper angle is reduced as described above, an insulation distance between the devices becomes short such that the breakdown voltage is reduced. This results in a reduction in the function of STI, that is, function of isolating elements adjacent to each other. Accordingly, in order that the aforementioned inconvenience may be prevented, the taper angle needs to be maintained at a large value. Thus, the conditions under which the opening width of the trench gives advantage to the trench fill capability is contrary to those under which the taper angle gives advantage to the trench fill capability. Under these conditions, the trench fill capability is hard to maintain at the level of the previous design rules.
In order that the trench fill capability of insulating film may be improved, for example, the trench filling step needs to be repeated several times, instead of forming the insulating film at a single step so that the trench is completely filled with the insulating film. In this case, however, the productivity cannot be improved even if the trench fill capability can be satisfied. Additionally, it is difficult to reduce the costs.